1. Technical Field
The present invention relates to a nonaqueous electrolyte secondary battery containing a positive electrode having a positive electrode active material capable of intercalating/deintercalating lithium ions; a negative electrode having a negative electrode active material capable of intercalating/deintercalating lithium ions; and a nonaqueous electrolyte in which an electrolyte composed of a lithium salt is dissolved in a nonaqueous solvent.
2. Related Art
Recently, downsizing and weight saving of a mobile information terminal such as a mobile telephone, a notebook computer, and a PDA are rapidly developing. Under such a background, a nonaqueous electrolyte secondary battery having a high energy density and a high capacity and represented by a lithium battery has been widely utilized as a driving power source for such a mobile information terminal. Such a nonaqueous electrolyte secondary battery is constituted usually using a positive electrode composed of a lithium-containing transition metal compound oxide such as LiCoO2, LiNiO2, LiMn2O4, LiFeO2, a negative electrode composed of carbonaceous materials such as graphite and a nonaqueous electrolyte in which an electrolyte composed of a lithium salt is dissolved in a nonaqueous solvent.
In such a nonaqueous electrolyte secondary battery, a nonaqueous electrolyte composed of a nonaqueous solvent and an electrolyte is used. As the nonaqueous solvent of the nonaqueous electrolyte, generally used is a solvent mixture in which a cyclic carbonate ester and a chain-shaped carbonate ester are mixed. Though with such a solvent mixture, excellent discharging properties have been obtained, oxidation resistance and reactivity with lithium are not necessarily stable, so that a disadvantage is caused that the safety of the battery is poor. Particularly, diethyl carbonate (DEC) has a disadvantage that it causes an uncontrollable exothermic reaction with lithium at around 90° C. Further, though dimethyl carbonate (DMC) has not exhibited such a high reactivity with lithium as that of DEC, DMC suffers from the disadvantage of having a low flash point and poor cycle performance.
On the surface of the material becoming a negative electrode active material of such a nonaqueous electrolyte secondary battery, a nonaqueous solvent becoming a component of the nonaqueous electrolyte becomes engaged and a side reaction affecting adversely the battery properties is caused. Therefore, for preventing a direct reaction of the negative electrode with the nonaqueous solvent, it has become an important task not only to form a coating film on the surface of the negative electrode, but also to control the formed state and the properties of the coating film. As a technology for controlling such a negative electrode surface coating film (SEI: solid electrolyte interface), adding a special additive into the nonaqueous electrolyte is generally known.
Here, for improving the overcharging performance, heretofore, an aromatic hydrocarbon such as cyclohexylbenzene (CHB) and biphenyl (BP) or a redox shuttle agent such as anisole have been added to the nonaqueous electrolyte. However, many of them have only unsatisfactory effect and for obtaining satisfactory effect, the added amount thereof is needed to be enlarged. When the added amount thereof is enlarged, a new disadvantage is caused that the high temperature retention characteristics and the cycle performance is significantly lowered.
Therefore, enlarging the adding ratio of a nonaqueous solvent having a high flash point such as ethylene carbonate (EC) and propylene carbonate (PC) and an improvement of using a new flame retardant nonaqueous solvent are attempted. However, since ethylene carbonate (EC), propylene carbonate (PC), and a flame retardant nonaqueous solvent have poor oxidation resistance, a disadvantage is caused that the retention characteristics and the cycle performance are lowered. Further, carboxylate esters have such an advantage that due to a strong oxidation resistance thereof, the reactivity thereof with the positive electrode becomes moderate. However, since they have a high reactivity with the negative electrode and consequently, are easily reduction-decomposed, the negative electrode coating film becomes unstable and they are poor in the cycle performance and the retention stability Therefore, it is difficult to use carboxylate esters as a nonaqueous solvent.
Further, tertiary carboxylate esters have a structure having high stability against an attack of a nucleophilic agent generated on the negative electrode by the increase of electron density of a carbonyl carbon and increase of steric hindrance due to a tertiary alkyl group thereof and further, by having no reactive active a hydrogen. As a result, they have higher thermal stability with a charging graphite negative electrode, metal lithium, lithium-Sn, Si alloy than that of a chain carbonate ester solvent. However, when they are used in a large amount, a disadvantage is caused that with respect to the cycle performance and the retention characteristics, a satisfactory performance cannot be obtained.
Therefore, it has been developed in WO 2002/015319 to stabilize the negative electrode coating film by mixing a cyclic carbonate ester or vinylene carbonate with tertiary carboxylate esters. Further, it has also been developed in JP-A-2003-59529 that by mixing divinylsulfon or alkine derivatives having a triple bond, the cycle performance and the high temperature retention characteristics can be improved. Further, it has also been developed in JP-A-2006-32301 that by mixing cyclic ester derivatives (such as FEC) having a halogen atom, the negative electrode is stabilized to improve the cycle performance. Further, it has also been developed in JP-A-2006-294373 to use a nonaqueous electrolyte containing succinic anhydride, glutaric anhydride, and diglycolic anhydride as a cyclic acid anhydride.
Here, in the nonaqueous electrolyte secondary battery developed in WO 2002/015319, since carboxylate esters or vinylene carbonate is mixed with tertiary carboxylate esters, the negative electrode coating film is stabilized and the cycle performance becomes improved. Further, in the nonaqueous electrolyte secondary battery developed in JP-A-2003-59529, by mixing divinylsulfon or alkine derivatives having a triple bond with the nonaqueous electrolyte, the cycle performance and the high temperature retention characteristics become improved. Further, in the nonaqueous electrolyte secondary battery disclosed in JP-A-2006-32301, by mixing cyclic ester derivatives (such as FEC) having halogen atoms, the negative electrode coating film is stabilized and the cycle performance becomes improved.
Further, in the nonaqueous electrolyte secondary battery developed in JP-A-2006-294373, since the nonaqueous electrolyte contains succinic anhydride, glutaric anhydride, and diglycolic anhydride as a cyclic acid anhydride, a decomposition reaction of the nonaqueous electrolyte is suppressed and consequently, the cycle performance becomes improved.
However, in the nonaqueous electrolyte secondary battery developed in the WO 2002/015319, since the coating film formation with a cyclic carbonate ester on the surface of the negative electrode is not satisfactory, the content of a tertiary carboxylate ester is confined within a range of 35% by mass or less and due to this small content, a disadvantage is caused that satisfactory cycle performance or retention characteristics cannot be obtained. Further, also in the nonaqueous electrolyte secondary battery developed in JP-A-2003-59529, the added amount of a tertiary carboxylate ester is in the range of 1 to 6% by mass, even at most around 20% by mass, so that a disadvantage is caused that satisfactory cycle performance or retention characteristics cannot be obtained.
Further, in the nonaqueous electrolyte secondary battery developed in JP-A-2006-32301, the cycle performance is improved by stabilizing the negative electrode coating film. However, since during retention at high temperatures and continuous charging, a generation of a large amount of gas is accompanied, a disadvantage is caused that satisfactory cycle performance or retention characteristics cannot be obtained. Further, in the nonaqueous electrolyte secondary battery developed in JP-A-2006-294373, the nonaqueous electrolyte contains succinic anhydride, glutaric anhydride, and diglycolic anhydride as a cyclic acid anhydride. However, since such a cyclic acid anhydride is contained in a cyclic carboxylate ester having a halogen atom, a disadvantage is caused that satisfactory cycle performance or retention characteristics cannot be obtained.